Static Electricity

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What is Electricity?

• Benjamin Franklin recognized that there are two opposite types of electric charge and named them positive and negative.

• A simple rule to decide the interaction between two separate charges is - like charges repel, unlike charges attract.

• The phenomenon of electricity was recognized in Ancient Greece nearly 5,000 years ago, but was not understood completely until the 20th century when the electrical model of the atom was developed.

Electrical model of atom

• There are three types of particles found in atoms.

1. The proton carries the positive charge 2. The electron carries the negative charge 3. The neutron is neutral (uncharged).

Each atom has a nucleus consisting of protons and neutrons.

- The electrons are some distance away from the nucleus, and are held loosely to the atom. Atoms are largely empty space.

- An ion is an atom which carries an excess charge.

- A positive ion is an atom or molecule that has lost one or more of its electrons, leaving it with a net positive charge.

- A negative ion is an atom or molecule that has gained one or more electrons, leaving it with a net negative charge.

Properties of the three atomic particles

Particle Relative Charge

Charge (C) Mass (kg)

Proton +1 (+e) +1.60 x 10-19 1.66 x 10-27 Electron -1 (-e) -1.60 x 10-19 9.11 x 10-31

Neutron 0 0.00 1.67 x 10-27

The Coulomb (C) is the SI unit of electric chargee : elementary charge

Note:

• Proton and electron have equal, but opposite charges.

• A Neutral object has the same number of protons and electrons.

• The proton is nearly 2,000 times more massive than the electron and is tightly bound in the nucleus (along with neutrons)

Example

• A glass rod becomes positively charged when it is rubbed with silk. Explain how this occurs.

– The glass rod loses electrons to the silk, which becomes negatively charged.

The principle of conservation of charge states:

• The net electric charge in an isolated system remains constant.

• Electric charge cannot be created or destroyed – Electric charge is conserved.

Pair work/ home work

• Text book : Read pgs 540-545• Pg. 545 Do Section Review 1-8

Insulators and conductors

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Conductors and Insulators

• Most materials can be categorized as conductors or insulators.

• Insulators greatly restrict the flowing of electrons through the material. Examples are glass, rubber, wood and plastics.

• Conductors permit the flowing of electrons through the material. examples are metals, many liquids, and plasmas.

• Semiconductors are intermediate in their ability to conduct charge.

• A superconductor is a conductor, which at low temperatures, permits currents to pass unimpeded through the material.

• Positive and negative charges can be separated by rubbing two objects together.

• Both insulators and conductors can become charged by contact.

• Conductors can be charged by induction, which is a process that causes charges to separate without touching the object.

• A surface charge can be induced on an insulator by polarization, which results in more positive charge on one side of a molecule than on the other side.

One last comment on conductors

• The Earth is an electrical conductor and can accept of donate large numbers of electrons.

• A charged object placed in contact with the Earth loses its own charge to the Earth. – Earth remains essentially neutral because of size

• This is known as grounding• Silly -

http://phet.colorado.edu/simulations/sims.php?sim=John_Travoltage

Brainiac’s video

• http://www.youtube.com/watch?v=kwaRgp7ycf0

Elementary Charge

• The magnitude of the charge on a proton or an electron is 1.60 x 10-19 coulomb

• This quantity is known as the elementary charge and is denoted by the letter e

Q = neQ : charge on the object (in coulombs)n: # of elementary chargese: the elementary charge

Example

• A balloon has acquired a charge of -3.20 x 10-17C. How many excess electrons does this charge represent?

Q = ne -3.20 x 10-17C. = n (-1.60 x 10-19 C)

n = 200. excess electrons

Example

• How many elementary charges are present in 1.00 C of charge?

Q = ne 1.00 C. = n (-1.60 x 10-19 C)

n = 6.25 x 1018 elementary charges

Coulomb’s Law

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Electric Force

• Coulomb's law is the magnitude of the force F between two charges. The symbol for electric charge is Q. Charges are assumed to be point charges.

Fe = k q1q2/r2

k : Electrostatic (or Coulomb) constant: 9 x109 Nm2 /C2 . q1q2 : the two point charges (C)

r2: the distance between the charges

Note:

Coulomb's law closely resembles Newton's law of gravitation.

Fg= G m1m2/r2

G: 6.67 x 10-11 Nm2/kg2 (very tiny!)k: 8.99 x 109 Nm2/C2 (very large!)

• The law of gravitation is dominant in systems on larger scales, like planets and stars.

• Coulomb's law is dominant in systems on smaller scales, like atoms, molecules, liquids, and solids.

• The electric force is a vector quantity - Coulomb's law is the magnitude of the force between any two charges.

• If only two charges are present in the system, the net force is directed along the line connecting the two charges

• If there is more than two charges present in the system, the direction of the net force can be calculated by the component method of vector addition

example

a) Calculate the magnitude of the force between two positive charges, q1 = 3.0 x 10-6 C and q2= 6.0 x 10 -5 C, separated by a distance of 9.0 meters.

b) Draw a diagram representing this situation.

solution

(a) Use Coulomb’s Law: Fe = k q1q2/r2

= = 2.0 x 10-2 N

2

562

29

)0.9(

)100.6)(100.3(100.9

m

CCCmN

(b) Diagram+3.0 x 10-6 C +6.0 x 10-5 C

+q1 +q2

Fe Fe

r

9.0m

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Hw

• Text Book – Read pgs 546 – 553• Pg 552-553 Do Practice problems 9-13 odd

and Section Review 15-21 odd• Review book read pgs 114-116 – Do pg 116 – 118 # 1-17

The Electroscope

The Braun Electroscope

• A device for detecting the presence of electric charge.

• Consists of a flat plate, a vertical post, and a “leaf”, all of which are conductors.

• There’s also a circular shield, which prevents stray charges from affecting the electroscope.

• The shield is separated from the rest of the device by an insulating collar placed under the plate

Electric Fields

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Electric Fields

• A field is a region of space in which a certain quantity has a definite value at every point.

Examples of common fields are gravitational fields, electric fields, and magnetic fields.

• The electric field around a fixed charge Q is:

E = Fe / Qo or E= k Q/r2

- The electric field is measured in N/C.

- When more than one charge contributes to the electric field, the net field E is the vector sum of the field contributions from the individual charges.

- Electric field lines provide a way for us to visualize the electric field surrounding a charge or a system of charges.

Drawing Field Lines

Three rules for drawing electric field lines are: 1. Electric field lines leave positive charges,

and enter negative charges. 2. The density of the electric field lines is

proportional to the magnitude of the field strength.

3. Field lines from the same field can not cross each other

• Although electric field lines do not actually exist, they help us to more completely understand the concept of fields.

• The electric field is radially outward from positive charges, and radially inward on negative charges.

• Field lines never intersect.

In the case of a charged metal object:

1. Inside the object the electric field is everywhere zero.

2. All excess charges reside on its surface. 3. The electric field is perpendicular to the

object's surface. 4. On irregularly shaped conductors

charges tend to accumulate at sharp points.

Two Positively Charged Objects

http://www.regentsprep.org/Regents/physics/phys03/alightnin/default.htm

lightning

HW

Read Pg 118 and 119 Do pg 120 #18-28

Recap: Electric Field Strength

• Measured by taking a very small positive test charge, placing it in the field and measuring the force on it.

• Vector quantity – in the direction of the force on the (positive) test charge

E = F / q• Unit is the newton per coulomb ( N/C )

Problem

• A test charge of + 2.0 x 10 -6 C experiences a force of 2.4 x 10 -3 N [E] when placed in an electric field. Determine the magnitude and direction of the electric field strength.

E = F / q = (2.4 x 10 -3 N [E] )

(2.0 x 10 -6 C)

= 1.2 x 10 3 N/C [E]

Potential Difference

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Potential Difference

• When a test point, q0, is moved between two points, A and B, in an electric field, if the charge is repelled by the field, work must be done to move the charge between the two points.

• Work done against the field (WAB) will increase the potential energy of the test charge.

Potential Difference (cont’d)

• Another way of describing this situation is to say that a potential difference exists between point A and B in the electric field.

• Potential Difference V = W/q • Scalar (as is work)• Unit is the joule per coulomb (J/C) called the

volt (V) in honor of Alessandro Volta (an Italian scientist)

Example 2

• When a charge of -4 x 10 -3 C is moved between two points in an Electric field, 0.8 J of work is done on the charge. What is the potential difference between the two points?

• Solution V = W / q = 0.8 J .

4 x 10 -3 C = 200 V

Note: Don’t care about sign only want magnitude

Example 3

• Calculate the work done on an elementary charge that is moved between two points in an electric field with a potential difference of 1V.

Rearrange eqn. W = qV = (1.6 x 10 -19 C) (1.0V) = 1.6 x 10 -19 J

This unit of energy is frequently used in Nuclear and Atomic Physics. This is an electron – volt (eV).

Electric Potential

• Need to establish a reference point of 0 V.• For an isolated charge:– reference point is infinitely far from the charge

• Ground may be taken as a reference point

• The electric potential at a point is defined as the work needed to move a charge of +1 C from infinity to the point in question.

Millikan Oil Drop Experiment• http://www.dnatube.com/video/2801/Millikan-

Oil-Drop-Experiment• By changing the potential difference between

the plates, the electric field strength was varied until the upward electric force on the droplet was balanced by the weight of the droplet.

• Millikan was then able to calculate the electric charge on each oil droplet he observed.

• By measuring thousands, he determined that the charges were all multiples of 1.60 x 10-19 C

• Thus concluded that the smallest charge, the Elementary charge is equal to 1.6 x 10 -19 C

Capacitance

• Read in text pg 577 – 579• Do question 16 to end on packet